Collective Diffusion of Gold Clusters and F-Centers at MgO(100) and CaO(100) Surfaces

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Few-atom gold clusters get trapped at the F-centers of MgO(100) and CaO(100) surfaces quickly upon deposition due to much stronger binding at the defects compared to the O-5c sites of the ideal terraces. Yet, our density functional theory (DFT) calculations reveal that their mobility is not fully suppressed since the Au dimers and trimers can diffuse at CaO(100) surface together with the F-centers they are bound to, along paths with activation energies not higher than 1.0 eV. The low energy paths are enabled by combined effects of high electron affinity of Au, the modest strength of the bonds within Au clusters, and a favorable topology of the point-defect electronic states along the paths. For other metals, the same diffusion mechanism is less effective than for gold.